Drills of the type to which the invention relates, and which by those skilled in the art are denominated short hole drills, are most commonly used for blast drilling of holes in workpieces of metal, such as steel, aluminum or the like. However, the drills may also work in other materials than metal.
Generally, the drills include a basic body as well as two bits, which are detachably mounted in pockets in the front end or tip of the basic body. Usually, the basic body is formed with a rear fastening part and a front, long narrow shank of a cylindrical basic shape. In the shank, two chip channels are formed, which advantageously extend helicoidally rearwards from the bits, which consist of on one hand a center bit which may be received in a center pocket positioned in the immediate vicinity of the geometrical center axis of the shank, and on the other hand a periphery bit which is mountable in a periphery pocket in the immediate vicinity of the peripherical envelope surface of the shank. Said bits are located substantially diametrically opposite each other on both sides of the geometrical center axis and arranged so that the operating ranges for their operative cutting edges overlap each other. More precisely, in the workpiece, the center bit is arranged to machine a circular area, the outer diameter of which is somewhat larger than the inner diameter of an imaginary ring-shaped area that is described by the periphery bit. The diameter of the hole (the drill diameter) drilled in the workpiece is determined by the radial distance between the center axis and a surface generating edge on the periphery bit.
Drills of the type in question are frequently available in sets that enable drilling of holes having different sizes, the individual set being based on use of one and the same type of center bit and periphery bit, respectively. For instance, an individual set may include a certain number of, e.g. 4-6, drills, having diameter differences of 1 mm, e.g. for drilling holes having 16 mm, 17 mm, 18 mm, 19 mm and 20 mm, respectively, diameter. In this connection, in order not to have to manufacture different types of bits, a manufacturing method is applied, according to which the different drills in a given set are given different active drill diameters by the fact that the periphery pockets, which all are similar in order to be able to receive one and the same type of bits, are placed differently far from the center axis of the drill.
A drawback of previously known manufacturing methods of the type mentioned above is that only the periphery bit, more precisely the pocket for this bit, is moved out with increasing drill diameter, but not the center pocket and the bit therein. This fact means that the relation between the effective cutting edge length of the center bit and the effective cutting edge length of the periphery bit will vary from drill to drill. The variation of the mutual relations between the two effective cutting edge lengths of the bits entails a plurality of disadvantages. Thus, one disadvantage is that the balance of the drill is negatively effected in the relative cutting edge length determining the forces that are applied to the shank via the two bits. Furthermore, the variation gives rise to unfavorable input or entering processes, among other things so far that the individual drill may enter the workpiece in a point that differs from the nominally intended, desired entering point. At four-edged bits, i.e., bits having four similar cutting edges, which enables four edges on one and the same bit to be indexed up to operative positions, there is furthermore a problem in previously known drills to protect the individual, inoperative surface-generating edge until the appurtenant cutting edge has been indexed up to an operative position.